This invention relates to a noncontact distance measuring system comprising a sensor that can be biased by alternating current and is provided with a measuring coil, an electronic supply/evaluation circuit, and an electrically and/or magnetically conductive test object, the measuring coil being packaged in a preferably cylindrical coil housing, and the test object surrounding the coil housing at least in part and being movable in its longitudinal direction. The present invention also relates to a method of noncontact distance measuring by means of a corresponding distance measuring system.
Noncontact distance measuring systems of different types and designs have been known in practice for years. By their basic method of operation, they may be classified on the one hand as distance measuring systems operating on the basis of eddy currents, inductive and capacitive distance measuring systems, and on the other hand as optical or acoustical distance measuring systems.
The present invention relates to a noncontact distance measuring system with a sensor having at least one coil, i.e., to distance measuring systems which operate either on the basis of eddy currents or by inductance.
Known already per se from DE-A 38 01 828 is a displacement sensor, in which a permanent magnet is arranged on a living body, and the change in the flux of the permanent magnet is measured as a measurement for the movement or vibration of the living body. In this prior document, the displacement sensor comprises a housing and a nonmagnetically conductive screening capsule inserted into the housing. Also the screening capsule is a dielectric, thermoplastic plastic. Furthermore, several induction coils are provided inside the housing. The screening capsule serves primarily not to disturb or influence a processing circuit by electromagnetic or electric fields of the displacement sensor.
Further known per se from DE-B-1 900 894 is a method of making an inductive transducer for measuring a distance. This cited publication also discloses sensors, which are surrounded on the one hand by a metal sleeve, and protected on the other hand by an epoxy resin applied to their measuring side.
Further known already from practice are so-called eddy-current long-distance measuring sensors, which are used, for example, in hydraulic or pneumatic cylinders for measuring the displacement and position of pistons or valves. Furthermore, long-distance measuring sensors of the kind in question permit a displacement measuring on presses, punches, roll stands, etc. In hydraulic and pneumatic cylinders, the sensors are packaged in pressure-resistant manner, and the housings are made of stainless steel. The eddy-current measuring principle applied therein is noncontacting. Thus, the sensors are not subjected to mechanical wear. Used as test object is an aluminum pipe which is moved concentrically and free of contact over a bar. Arranged in the interior of the bar is a coil which is protected against environmental influences. By the inductance of eddy currents in the aluminum pipe, energy is withdrawn from the coil and it is thus detuned.
An integrated, miniaturized electronic circuit converts the pipe position into a linear electronic output signal ranging normally from 4 to 20 mA. To supply the sensor, a dc voltage between 15 and 30 volts is provided. The long-distance measuring sensors known from practice are however problematic, inasmuch as the aluminum pipe serving as test object is pushed as a whole over the bar which accommodates the coil. Thus, if the entire length of the coil is to be used as length of measurement, it will be necessary to push the aluminum pipe almost entirely over the bar, so that from the length of the bar and of the aluminum tube, an approximately resultant overall length of the long-distance measuring sensor or the distance measuring system at issue is obtained.
Moreover, there exist problems to the extent that in the case of the long-distance measuring sensors in question, the output impedance of the measuring coil is always dependent on the position of the test object or of the aluminum pipe serving as test object. Consequently, it is necessary in this instance to compensate by providing a corresponding electronic system, which results in enormous costs with regard to design and construction.
It is therefore the object of this invention to describe a noncontact distance measuring system, which comprises a measuring coil having a small overall length and being of a simple construction, the output impedance of which is independent of the position of the test object. Furthermore, it is the object to describe a corresponding method of noncontact distance measuring.